Sn-ag based lead-free solder

ABSTRACT

To provide a tin-silver based lead-free solder which can improve joint strength in the initial stage, and can curtail a decrease in joint strength after heat treatment, at a joined interface with an electroless plating, zinc (Zn) is added to a solder comprising a tin-silver system containing no lead.

TECHNICAL FIELD

This invention relates to a solder comprising a tin-silver systemcontaining no lead.

BACKGROUND ART

In recent years, electronic devices have increasing becomenarrow-pitched in response to the downsizing and high functionality ofvarious electric equipment. In electronic mounting and packagingtechnologies, electroless Ni—P plating, which is capable of selectiveprecipitation and can form a uniform film, is applied.

From the aspect of global environmental issues in recent years, on theother hand, solders containing no lead (lead-freed) as a constituentelement have promptly advanced. In light of various characteristics,Sn—Ag based solders are considered to be the closest to practical use(see, for example, Japanese Patent Application Laid-Open No.1998-328880, Japanese Patent Application Laid-Open No. 2001-71174, andJapanese Patent Application Laid-Open No. 2001-150181).

In recent years, a two-element solder, Sn-3.5Ag solder, has begun to beused for BGA (Ball Grid Array) This Sn-3.5Ag solder has the problemsthat at its interface with an electroless Ni—P plating, joint strengthin an initial stage is low, and joint strength after heat treatmentdecreases.

Moreover, the Sn-3.5Ag solder has a high melting point. Thus, packagingevaluation in recent years has shown this solder to cause damage tocomponents under the influence of heat. In view of this finding, theaddition of In, an element effectively lowering the melting point, hasbeen studied. Since In, when added, scarcely changes the mechanicalproperties of the solder itself, it has attracted attention in recentyears.

The Sn-3.5Ag solder, however, poses the following problems: If theamount of In added increases, the joint strength in the initial stagedecreases at its joined interface with the electroless Ni—P plating. Ifthe amount of In added is 5% or more, in particular, a decrease in thejoint strength after heat treatment is very great.

It is an object of the present invention, therefore, to provide atin-silver based lead-free solder which can improve joint strength inthe initial stage at a joined interface between Sn-3.5Ag or Sn-3.5Ag-xInand an electroless plating, and can curtail a decrease in the jointstrength after heat treatment.

DISCLOSURE OF THE INVENTION

We, the inventors, diligently conducted studies to solve theabove-described problems. As a result, we have found that the additionof a small amount of zinc to a tin-silver based solder composition iseffective for the strength of joint, and a change in an interface layer,with an electroless Ni—P plating This finding has led us to accomplishthe present invention.

A first invention based on this finding lies in a tin-silver basedlead-free solder, characterized by containing zinc (Zn) added to asolder comprising a tin-silver system containing no lead.

A second invention lies in the tin-silver based lead-free solder of thefirst invention, characterized by further containing indium (In) addedthereto.

A third invention lies in the tin-silver based lead-free solder of thefirst invention, characterized in that the amount of zinc (Zn) added is0.3 to 1.0 wt. %, the remainder being tin and silver.

A fourth invention lies in the tin-silver based lead-free solder of thesecond invention, characterized in that the amount of indium (In) addedis less than 10 wt. %, and the amount of zinc (Zn) added is 0.1 to 1.0wt. %, the remainder being tin and silver.

A fifth invention lies in a joint structure, characterized in thatbodies to be joined are joined together by the tin-silver basedlead-free solder of any one of the first to fourth inventions.

A sixth invention lies in the joint structure of the fifth invention,characterized in that an electroless plating layer is provided on thesurface of the bodies to be joined.

A seventh invention lies in the joint structure of the sixth invention,characterized in that the electroless plating layer is a Ni—P plating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configurational drawing of a test piece used in anExample of the tin-silver based lead-free solder according to thepresent invention.

FIG. 2 is a graph showing changes with time in the thickness of a Ni—Snreaction layer upon heat treatment of a test piece using a solder of acomposition A1.

FIG. 3 is a graph showing changes with time in the thickness of a Ni—Snreaction layer upon heat treatment of a test piece using a solder of acomposition B1.

FIG. 4 is a graph showing changes with time in the thickness of a Ni—Snreaction layer upon heat treatment of a test piece using a solder of acomposition C5.

FIG. 5 is a graph showing changes with time in the thickness of aP-concentrated layer upon heat treatment of the test piece using thesolder of the composition A1.

FIG. 6 is a graph showing changes with time in the thickness of aP-concentrated layer upon heat treatment of the test piece using thesolder of the composition B1.

FIG. 7 is a graph showing changes with time in the thickness of aP-concentrated layer upon heat treatment of the test piece using thesolder of the composition C5.

FIG. 8 is a graph showing changes with time in the thicknesses of Ni—Snreaction layers upon heat treatment of test pieces using solders ofcompositions A1, D2, D4 and D6.

FIG. 9 is a graph showing changes with time in the thicknesses ofP-concentrated layers upon heat treatment of the test pieces using thesolders of the compositions A1, D2, D4 and D6.

FIG. 10 is a graph showing changes with time in the thicknesses of Ni—Snreaction layers upon heat treatment of test pieces using solders ofcompositions D6 and E1 to E5.

FIG. 11 is a graph showing changes with time in the thicknesses ofP-concentrated layers upon heat treatment of the test pieces using thesolders of the compositions D6 to E1 to E5.

FIG. 12 is a graph showing the relationship between the amount of zincadded and wettability in a Sn—Ag—In—Zn solder.

FIG. 13 is a SEM photograph of interface layers after heat treatment ofthe test pieces, the solder of the composition D6 being used in FIG.13(a), and the solder of the composition E5 being used in FIG. 13(b).

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the tin-silver based lead-free solder according to thepresent invention will now be described, but the present invention isnot limited to the following embodiments.

The tin-silver based lead-free solder according to the present inventionis a solder comprising a tin(Sn)-silver(Ag) system containing no lead,the solder having zinc (Zn) added thereto.

The tin-silver based lead-free solder contains 3 to 3.5 wt. % of silver(Ag), the remainder being tin (Sn). The solder containing 3.5 wt. % ofsilver (Ag) blended with tin (Sn) is designated as Sn-3.5Ag.

The amount of zinc (Zn) added is preferably 0.3 to 1.0 wt. %, and morepreferably 0.7 to 1.0 wt. %. This is because if the amount of zinc (Zn)added is less than 0.3 wt. %, the effect of improving the joint strengthin an initial stage is low. If the amount of zinc (Zn) added exceeds 1.0wt. %, the effect of improving the initial joint strength is low, andthe effect of curbing the decrease in the joint strength after heattreatment is also low.

In the tin-silver based lead-free solder according to the presentinvention, it is also preferred for indium (In) to be further added.

At this time, it is preferred that the amount of indium (In) added isless than 10 wt. %, and the amount of zinc (Zn) added is 0.1 to 1.0 wt.%. This is because the amount of indium (In) added being 10 wt. % ormore is not preferred from the aspects of cost and solderability. Theamount of zinc (Zn) added being less than 0.1 wt. % would result in agreater decrease in the joint strength. The amount of zinc (Zn) addedexceeding 1.0 wt. % would lead to decline in wettability.

When bodies to be joined are joined together by use of theabove-described tin-silver based lead-free solder according to thepresent invention, the joining strength of the junction structure isimproved.

Particularly when an electroless plating layer is provided on the bodiesto be joined, the decrease in the joint strength can be markedlycurtailed in comparison with the conventional solder.

The type of the electroless plating layer is not limited, but a Ni—Pplating is extremely effective.

Example

To confirm the effect of the tin-silver based lead-free solder accordingto the present invention, the following tests were conducted:

[Preparation of Solder]

The components were blended in the proportions shown in the Table 1indicated below. The blend was melted at 300° C., then poured into amold, and cooled to room temperature for casting, thereby preparing atin-based lead-free solder. TABLE 1 Composition Ag Cu In Zn Sn A 1 3.5 —— — Remainder B 1 3.5 0.5 — — Remainder C 1 3.5 — — 0.1 Remainder 2 3.5— — 0.3 Remainder 3 3.5 — — 0.5 Remainder 4 3.5 — — 0.7 Remainder 5 3.5— — 1.0 Remainder 6 3.5 — — 1.3 Remainder 7 3.5 — — 1.5 Remainder D 13.5 — 3 — Remainder 2 3.5 — 4 — Remainder 3 3.5 — 5 — Remainder 4 3.5 —6 — Remainder 5 3.5 — 7 — Remainder 6 3.5 — 8 — Remainder 7 3.5 — 10  —Remainder E 1 3.5 — 8 0.1 Remainder 2 3.5 — 8 0.3 Remainder 3 3.5 — 80.5 Remainder 4 3.5 — 8 0.7 Remainder 5 3.5 — 8 1.0 Remainder[Preparation of Body to be Joined]

As shown in FIG. 1(a), an electroless Ni-10P plating layer 11 b(thickness: about 5 μm) was applied to a copper plate 11 a (10×30×1 mm)to prepare a body 11 to be joined.

[Preparation of Test Piece]

As shown in FIG. 1(b), two of the aforementioned solders 12 (2×2×0.1 mm)in the form of a thin piece were interposed between two of theabove-mentioned bodies 11 to be joined. The resulting composite washeated (250° C.×40s) by a hot plate to join the bodies 11 together,whereby a test piece 10 was prepared for the aforementioned solder 12 ofeach of the aforesaid compositions A1, B1, C1 to C7, D1 to D7, and E1 toE5.

[Testing Methods]

The joint strengths of each of the test pieces 10 before heat treatmentand after heat treatment were measured by an Instron type tester(measurement conditions: temperature=room temperature, crossheadspeed=10 mm/min, n=5).

The joined interface was observed by a scanning electron microscope(SEM) and an energy dispersive X-ray analyzer (EDX) to investigatechanges with time in the thickness of the interface layer due to heattreatment (arbitrary 10 points were sampled from a photograph takenunder SEM, measurements were made of these points, and the average ofthe measured values was calculated).

[Test Results]

(1) Compositions A to C

i) Joint Strength

The results of the measurements of the joint strength of the test piece10 using the solder 12 of each of the compositions A to C are shown inTables 2 and 3 below. The heat treatment was performed for 1,000 hoursat temperatures of 100° C., 125° C. and 150° C. TABLE 2 Heat BeforeAfter treatment heat heat temperature treatment treatment DecreaseComposition (° C.) (MPa) (MPa) rate (%) A1 100 35.73 33.73 5.6 125 35.7332.96 7.8 150 35.73 30.80 13.8 B1 100 37.56 34.28 8.7 125 37.56 31.3716.5 150 37.56 29.80 20.7

TABLE 3 Heat Before After treatment heat heat temperature treatmenttreatment Decrease Composition (° C.) (MPa) (MPa) rate (%) C1 100 35.3833.61 5.0 125 35.38 32.80 7.3 150 35.38 30.56 13.6 C2 100 35.86 34.104.9 125 35.86 33.42 6.8 150 35.86 31.12 13.2 C3 100 36.15 34.80 3.7 12536.15 33.91 6.2 150 36.15 31.56 12.7 C4 100 36.52 35.80 2.0 125 36.5234.31 6.1 150 36.52 32.20 11.8 C5 100 37.07 36.81 0.7 125 37.07 36.202.3 150 37.07 35.10 5.3 C6 100 35.59 34.90 1.9 125 35.59 33.48 5.9 15035.59 31.60 11.2 C7 100 34.28 33.42 2.5 125 34.28 31.90 6.9 150 34.2829.93 12.7

As shown in Tables 2 and 3, the test pieces 10 using the solders 12 ofthe composition B1 (copper added) and the compositions C2 to C5 (0.3 to1.0 wt. % of zinc added) improved in the joint strength in the initialstage over the test piece 10 using the solder 12 of the composition A1(no copper or zinc added).

The test pieces 10 using the solders 12 of the compositions C2 to C5(0.3 to 1.0 wt. % of zinc added) showed the effect of curtailing thedecrease in the joint strength after heat treatment, as compared withthe test piece 10 using the solder 12 of the composition A1 (no copperor zinc added). The test piece 10 using the solder 12 of the compositionC5 (1.0 wt. % of zinc added), in particular, was able to markedlycurtail the decrease in the joint strength after heat treatment. Thetest piece 10 using the solder 12 of the composition B1 (copper added),however, was unable to curtail the decrease in the joint strength afterheat treatment.

From the above findings, it has become clear that the solders 12 of thecompositions C2 to C5 (0.3 to 1.0 wt. % of zinc added) can improve thejoint strength in the initial stage, and curtail the decrease in thejoint strength after heat treatment, at the joined interface with theelectroless Ni-10P plating, and the solder 12 of the composition C5 (1.0wt. % of zinc added), in particular, shows this effect markedly.

ii) Changes in Thickness of Interface Layer

Generally, it is assumed that two types of layers, i.e., a Ni—Snreaction layer and a P-concentrated layer, are formed as an interfacelayer between a Sn—Ag based solder and an electroless Ni—P plating, andthese layers gradually grow upon heat treatment, whereby the jointstrength gradually lowers. Thus, the test pieces 10 using the solders 12of the composition A1 (no addition), the composition B1 (copper added),and the composition C5 (1.0 wt. % of zinc added) were examined forchanges with time in the thickness of the above interface layer uponheat treatment (temperatures of 100° C., 125° C. and 150° C.). Theresults are shown in FIGS. 2 to 7. In FIGS. 2 to 7, the horizontal axisrepresents the square root of the heat treatment time, and the verticalaxis represents the thickness of the above-mentioned layer.

As shown in FIGS. 2 to 7, the interface layer increases in thicknesslinearly, and is thus presumed to grow with rate-determining diffusion.

The changes with time in the thickness of the above interface layer uponheat treatment in the test piece 10 using the solder 12 of thecomposition C5 (1.0 wt. % of zinc added) are clearly smaller than thechanges with time in the thicknesses of the above interface layers uponheat treatment in the test pieces 10 using the solders 12 of thecomposition B1 (copper added) and the composition C5 (1.0 wt. % of zincadded).

Based on these findings, it is speculated that the addition of zincsuppresses the growth of the interface layer, thereby curtailing thedecrease in the joint strength after heat treatment.

(2) Composition D

i) Joint strength

The results of the measurements of the joint strengths of the testpieces 10 using the solders 12 of the compositions D1 to D7 (indiumadded) are shown in Table 4 below. As a control, the results of themeasurement of the joint strength of the test piece 10 using the solder12 of the composition A1 (no addition) are also tabulated. The heattreatment was performed for 1,000 hours at a temperature of 100° C.TABLE 4 Before After heat heat treatment treatment Decrease Composition(MPa) (MPa) rate (%) A1 35.73 32.92 7.9 D1 36.87 35.20 4.5 D2 36.5033.79 7.4 D3 36.25 28.55 21.2 D4 34.57 19.96 42.3 D5 34.09 18.41 46.0 D629.20 13.03 55.4 D7 24.48 15.40 37.1

As shown in Table 4, the test pieces 10 using the solders 12 of thecompositions D4 to D7 (6 wt. % or more of indium added) decreased in thejoint strength in the initial stage, as compared with the test piece 10using the solder 12 of the composition A1 (no addition) The test pieces10 using the solders 12 of the compositions D3 to D7 (5 wt. % or more ofindium added) decreased in the joint strength after heat treatment, ascompared with the test piece 10 using the solder 12 of the compositionA1 (no addition). The test piece 10 using the solder 12 of thecomposition D6 (8 wt. % of indium added), in particular, decreased inthe joint strength after heat treatment by as much as 55%.

ii) Changes in Thickness of Interface Layer

The test pieces 10 using the solders 12 of the compositions D2, D4 andD6 (4 wt. %, 6 wt. % and 8 wt. % of indium added) were examined forchanges with time in the thickness of the above interface layer uponheat treatment (100° C.). The results are shown in FIGS. 8 and 9. As acontrol, changes with time in the thickness of the above interface layerupon heat treatment (100° C.) in the test piece 10 using the solder 12of the composition A1 (no addition) are also shown.

As shown in FIGS. 8 and 9, the changes with time in the thickness of theabove interface layer upon heat treatment in the test piece 10 using thesolder 12 of the composition D2 (4 wt. % of indium added) werecomparable to the changes with time in the thickness of the aboveinterface layer upon heat treatment in the test piece 10 using thesolder 12 of the composition A1 (no addition). However, the changes withtime in the thicknesses of the above interface layers upon heattreatment in the test pieces 10 using the solders 12 of the compositionD4 and D6 (6 wt. % and 8 wt. % of indium added) were greater than thechanges with time in the thickness of the above interface layer uponheat treatment in the test piece 10 using the solder 12 of thecomposition A1 (no addition).

Based on these findings, it is speculated that when the amount of indiumadded exceeds a predetermined value, the growth of the interface layeris promoted, whereby the joint strength after heat treatment isdecreased.

(3) Composition E

i) Joint Strength

Table 5 offered below shows the results of the measurements of the jointstrengths of the test pieces 10 using the solders 12 of the compositionsE1 to E5 containing zinc added to the composition D6 (8 wt. % of indiumadded) that caused the largest decrease in the joint strength. As acontrol, the results of the measurement of the joint strength of thetest piece 10 using the solder 12 of the composition D6 (8 wt. % ofindium added) are also tabulated. The heat treatment was performed for1,000 hours at a temperature of 100° C. TABLE 5 Before After heat heattreatment treatment Decrease Composition (MPa) (MPa) rate (%) D6 29.2013.03 55.4 E1 33.68 28.00 16.9 E2 33.61 30.05 10.6 E3 34.17 34.30 0.0 E434.24 34.10 0.4 E5 35.75 34.99 2.1

As shown in Table 5, all the test pieces 10 using the solders 12 of thecompositions E1 to E5, which had zinc added thereto, were able tocurtail the decreases in the joint strengths in the initial stage andafter heat treatment, in comparison with the test piece 10 using thesolder 12 of the composition D6 having no zinc added thereto. The testpieces 10 using the solders 12 of the compositions E3 to E5 (0.5 to 1.0wt. % of zinc added), in particular, were found to have a very higheffect of curtailing a decrease in the joint strength.

ii) Changes in Thickness of Interface Layer

The test pieces 10 using the solders 12 of the compositions E1 to E5(zinc added) were examined for changes with time in the thickness of theabove interface layer upon heat treatment (100° C.). The results areshown in FIGS. 10 and 11. As a control, changes with time in thethickness of the above interface layer upon heat treatment (100° C.) inthe test piece 10 using the solder 12 of the composition D6 (no additionof zinc) are also shown.

As shown in FIGS. 10 and 11, the interface layer increases in thicknesslinearly, and is thus presumed to grow with rate-determining diffusion.

The changes with time in the thicknesses of the above interface layersupon heat treatment in the test pieces 10 using the solders 12 of thecompositions E1 to E5, which had zinc added thereto, were clearlysmaller than the changes with time in the thickness of the aboveinterface layer upon heat treatment in the test piece 10 using thesolder 12 of the composition D6 without the addition of zinc. Thechanges with time in the thicknesses of the above interface layers uponheat treatment in the test pieces 10 using the solders 12 of thecompositions E3 to E5 (0.5 to 1.0 wt. % of zinc added), in particular,were much smaller than the changes with time in the thickness of theabove interface layer upon heat treatment in the test piece 10 using thesolder 12 of the composition D6 (without the addition of zinc). Theseresults were the same as those obtained for the amounts of zinc additionfound to produce the effect of curtailing the decrease in the jointstrength.

Based on these findings, it is speculated that the addition of zincsuppresses the growth of the interface layer, thereby curtailing thedecrease in the joint strength after heat treatment. Accordingly, it canbe said that the suppression of the growth of the interface layer is ofcrucial importance to the curtailment of the decrease in the jointstrength.

The relationship between the amount of zinc added and wettability isshown in FIG. 12. As shown in FIG. 12, the addition of zinc is found todecrease wettability. In consideration of wettability, therefore, it ispreferred to set the amount of zinc addition at 0.7 wt. % or less.

An SEM photograph of the aforementioned interface layer after heattreatment (100° C.×1,000 hours) of the test piece 10 using the solder 12of the composition D6 is shown in FIG. 13(a). An SEM photograph of theaforementioned interface layer after heat treatment (100° C.×1,000hours) of the test piece 10 using the solder 12 of the composition E5(1.0 wt. % of zinc added) is shown in FIG. 13(b). As shown in FIG. 13,it is clear that the addition of zinc can suppress the growth of theinterface layer, thus curtailing the decrease in the joint strength.

[Summary]

The results of the investigation of the strength of joint, and changesin the interface layer, between the Sn—Ag based solder and theelectroless Ni—P plating in the foregoing Example can be summarized asfollows:

(1) By adding 0.3 to 1.0 wt. % of zinc to Sn-3.5Ag, it was possible toimprove the joint strength in the initial stage, and curtail thedecrease in the joint strength upon heat treatment. The addition of 0.7to 1.0 wt. % of zinc, in particular, increased this effect remarkably.Growth of the interface layer was also suppressed by the addition ofzinc.

(2) The joint strength of Sn-3.5Ag-xIn, in the initial stage, decreasedwhen the amount of indium added was 8 wt. % or more. The joint strengthof Sn-3.5Ag-xIn, after heat treatment, decreased when the amount ofindium added was 5 wt. % or more, and decreased maximally when theamount of indium added was 8 wt. %. In the test pieces that causeddecreases in the joint strength, the growth of the interface layertended to be great.

(3) When zinc was added to Sn-3.5Ag-8In, the joint strength in theinitial stage was increased. When heat treatment was performed withoutaddition of zinc, the joint strength after heat treatment decreased byabout 55%. Upon addition of zinc, the decrease in the joint strengthafter heat treatment was curtailed. At this time, the addition of 0.1wt. % of zinc can curtail the decrease in the joint strength, but theaddition of 0.5 wt. % or more of zinc an obtain a greater effect.

(4) Sn-3.5Ag-8In, when heat treated, leads to a marked growth of theaforementioned interface layer, but if 0.5 wt. % or more of zinc isadded, can suppress the growth of the interface layer.

INDUSTRIAL APPLICABILITY

The tin-silver based lead-free solder of the present invention containszinc (Zn) added thereto, and thus can suppress the growth of theinterface layer, improve the joint strength in the initial stage, andcurtail the decrease in the joint strength after heat treatment.

Particularly, if bodies to be joined, which have been plated, are joinedtogether by the solder of the present invention, the joint strength inthe initial stage can be improved, and the decrease in the jointstrength after heat treatment can be curtailed.

1. A tin-silver based lead-free solder containing zinc (Zn) added to asolder comprising a tin-silver system containing no lead.
 2. Thetin-silver based lead-free solder of claim 1 further containing indium(In) added thereto.
 3. The tin-silver based lead-free solder of claim 1,containing 0.3 to 1.0% by weight of zinc (Zn) the remainder is tin andsilver.
 4. The tin-silver based lead-free solder of claim 2, containingless than 10% by weight of indium (In) 0.1 to 1.0% by weight zinc (Zn)and the remainder is tin and silver.
 5. A joint structure, comprisingbodies to be joined are joined together by the tin-silver basedlead-free solder of claim
 1. 6. The joint structure of claim 5, whereinan electroless plating layer is provided on surfaces of said bodies tobe joined.
 7. The joint structure of claim 6, whereinsaid_electroless_plating layer is a Ni—P plating.